This protocol investigates in vivo metabolism of liproteins in patients with homozygous familial hypercholesterolemia. A total of four subject were studied. They received a prime constant infusion of deuterated leucine. Lipoproteins are isolated and the enrichment of leucine in apolipoproteins are determined. Results thus far demonstrate that patients with homozyugous FH have a markedly delayed catabolism of apoB containing lipoproteins. In addition, they have decreased apoB production including directly into the LDL subfraction. This protocol is providing important new insights into lipoprotein metabolism in homozygous FH.